With increasing environmental consciousness, there are growing demands on clean energy, natural energy or renewable energy. For example, solar energy, wind power, water power, marine energy and tide energy are the well-known power sources of the clean energy, natural energy or renewable energy. As known, wind power is generated by using airflow (e.g. wind) to driving rotation of a wind wheel of a wind turbine and using a primary generator of the wind turbine to convert the rotating kinetic energy into electric power. Generally, the wind wheel comprises plural blades or airfoils.
Generally, the wind turbines are classified into two types, i.e. a horizontal axis wind turbine and a vertical axis wind turbine. During operations of the horizontal axis wind turbine, the central axis of the wind wheel is in parallel with the ground or the direction of the airflow. During operations of the vertical axis wind turbine, the central axis of the wind wheel is perpendicular to the ground or the direction of the airflow. As the direction of the airflow changes, for allowing the wind wheel to be rotated with the airflow, the orientation of the wind wheel of the horizontal axis wind turbine should be correspondingly changed. However, in response to the airflow in any direction, the wind wheel of the vertical axis wind turbine can be rotated. That is, even if the direction of the airflow changes, it is not necessary to adjust the orientation of the central axis of the wind wheel.
FIG. 1 is a schematic perspective view illustrating a conventional vertical axis wind turbine. As shown in FIG. 1, the vertical axis wind turbine 100 principally comprises a support axis 11, a primary generator 12, plural fixing props 13, and plural blades 14 corresponding to the fixing props 13. For clarification, three blades 14 are shown in the drawing. The plural blades 14 are fixedly disposed on the corresponding fixing props 13. Consequently, the blades 14 and the fixing props 13 are collaboratively constituted as a wind wheel. The wind wheel is rotatably disposed on the support axis 11. Upon rotation of the wind wheel, the support axis 11 is used as a central axis. Moreover, the support axis 11 is used for supporting the primary generator 12. The primary generator 12 may be disposed within the support axis 11, or located over or under the support axis 11. Consequently, the support axis 11 (i.e. the central axis of the wind wheel) is perpendicular to the ground or the direction of the airflow. As shown in FIG. 1, the direction of the airflow is denoted by the direction of an ambient wind speed v. Moreover, upon rotation of the wind wheel, the primary generator 12 is driven to generate electric power.
Generally, as shown in FIG. 1, the cross section of the blade 14 of the vertical axis wind turbine 100 has an airfoil configuration. According to the aerodynamic principle, the airflow around the airfoil may create a lift force. In response to lift force, a rotational torque is generated to rotate the wind wheel. Moreover, for achieving the both functions of generating a large rotational torque and supporting the weight of the blade 14 during operation of the wind wheel, the connecting point (i.e. the fulcrum P1) between the fixing prop 13 and the corresponding blade 14 is separated from a leading edge of the blade 14 by a specified distance. For example, the fulcrum P1 is separated from the leading edge of the blade 14 by one third or one fourth of a chord length.
The wind turbine 100 as shown in FIG. 1 is a wind turbine with vertical type (H-type) blades. It is noted that the shapes of the blades of the vertical axis wind turbine are not restricted. For example, the blades of other vertical axis wind turbines may have symmetric shapes, arc shapes, curvy shapes, helical shapes or other special shapes.
FIGS. 2A and 2B schematically illustrate two-dimensional expression of associated forces exerted on the blade of the vertical axis wind turbine of FIG. 1. As shown in FIG. 2A, a relative wind speed w is a wind speed relative to the blade 14 in the ambient wind speed v. An angle between the relative wind speed w and a chord line c of the blade 14 (i.e. the line passing through the leading edge and the trailing edge of the blade) is referred as an angle of attack (α).
According to aerodynamics, the magnitude of the angle of attack (α) is related to the magnitude of the lift force that is caused by the airflow and exerted on the blade 14. In case that the direction and speed of the ambient wind speed v is constant and the blade 14 is fixedly disposed on the corresponding prop 13, the direction of relative wind speed w on the blade 14 is unceasingly changed as the blade 14 is rotated. Consequently, during rotation of the blade 14, the magnitude of the angle of attack (α) is correspondingly changed. That is, as the blade 14 is rotated one turn, the azimuth angle varies from 0 to 360 degrees, and the lift forces corresponding to different angles of attack (α) are different. Consequently, the rotational torques for rotating the blade 14 about the vertical axis and corresponding to different azimuth angles will be different. In other words, it is impossible to control and maintain the optimal angle of attack to acquire the optimal lift force. For example, if the angle of attack is too large, the drag coefficient of the oncoming airflow will increase and lift coefficient of the oncoming airflow will decrease.
Moreover, the blade 14 is fixedly disposed on the corresponding prop 13, and the fulcrum P1 between the fixing prop 13 and the corresponding blade 14 is not located at the leading edge of the blade 14. Upon rotation of the tip of the fixing prop 13, the fulcrum P1 between the fixing prop 13 and the corresponding blade 14 is moved along a circular path (i.e. in a circular motion). That is, during the practical rotation, the leading edge of the blade 14 is not rotated along the tangential direction of the circular trajectory of the fulcrum P1. In particular, the leading edge of the blade 14 is rotated at a pitch angle γ (also referred as an angle of pitch).
As shown in FIG. 2A, the pitch angle γ is the angle between the moving direction m of the blade 14 (especially the leading edge of the blade 14) and the chord line c of the blade 14. The direction of the chord line c is in parallel with the tangential direction of the circular trajectory of the fulcrum P1. Under this circumstance, the magnitude of the lift force generated by the airflow around the blade 14 is dependent on the angle of attack (α). In addition, the positive or negative effect caused by the magnitude of the pitch angle γ may directly influence the change of the angle of attack (α) during rotation. On the other hand, if the fulcrum P1 between the fixing prop 13 and the corresponding blade 14 is changed, the pitch angle γ is correspondingly changed, and the angle of attack (α) is correspondingly influenced. Since the generated rotational torque is different, the rotating efficiency is different.
Moreover, as shown in FIG. 2B, the ambient wind speed v is the prevailing wind speed (i.e. in the direction of the airflow) in the ambient wind field. In FIG. 2B, the forces exerted on the blade 14 at the positions corresponding to two azimuth angles are shown. Generally, the azimuth angles from 0 to 180 degrees correspond to a windward side, and the azimuth angles from 180 to 360 degrees correspond to a leeward side. At the azimuth angle of 90 degrees, a lift force L1 caused by the relative wind speed w is exerted on the blade 14. The lift force L1 is perpendicular to the relative wind speed w. The lift force L1 may be resolved into two components. One component L1n is projected along the normal direction of the rotation plane, and the other component L1t is projected along the tangential direction of the rotation plane. Similarly, at the azimuth angle of 270 degrees, a lift force L2 caused by the relative wind speed w is exerted on the blade 14. The lift force L2 may be resolved into two components L2n and L2t. For clarification and brevity, the drag force (in parallel with the relative wind speed w) caused by the airflow and exerted on the blade 14 is not shown.
From the above discussions, each of the components L1t and L2t may exert a thrust force on the blade 14, thereby generating a rotational torque of rotating the blade 14. The component L1n at the windward side may exert a pressure on the support axis 11 (i.e. the central axis). The component L2n at the leeward side may exert a tension on the support axis 11. Since the direction of the pressure (i.e. the normal direction of the blade 14 toward the support axis 11) and the direction of the tension (i.e. the normal direction of the blade 14 away from the support axis 11) are the same as the direction of the ambient wind speed v (i.e. the direction of the airflow), the support axis 11 is pushed and pulled by the pressure and the tension, respectively. In other words, the support axis 11 is readily damaged.